1. Field of the Invention
The present invention relates to an optical scanning apparatus and, more particularly, to prevention of flare from a scanning mirror in the optical scanning apparatus used in printers in which an image is drawn with laser light.
2. Related Background Art
FIG. 8 is an optical path diagram of a conventional optical scanning apparatus. Reference numeral 91 designates a laser unit, 92 a cylindrical lens, 93 a scanning mirror, 94 a spherical lens, 95 a toric lens, and 96 a photosensitive drum. Parallel laser light emitted from the laser unit 91 is converged only in the sub-scanning direction by the cylindrical lens 92 to impinge on a surface of the scanning mirror 93. The scanning mirror 93 rotates at a constant speed and the light reflected by the scanning mirror 93 travels through the spherical lens 94 and the toric lens 95 to undergo correction for Fxcex8, whereby converging light scans the surface on the photosensitive drum 96. The photosensitive drum 96 rotates at a constant speed in synchronism with a driving signal of the semiconductor laser and the scanning light forms an electrostatic latent image on the photosensitive drum 96. An image is printed from this electrostatic latent image onto paper by the electrophotographic process.
Recently, there are strong desires for higher definition of image and higher speed of output and there are thus increasing needs to increase the width of the polygon mirror, i.e., to increase the size of the polygon mirror in order to make the Fxcex8 optical system brighter and, in addition thereto, to rotate the polygon mirror at a higher speed. On the other hand, there is a limit to the performance of the motor for rotating the polygon mirror of the thus increased size at the higher speed and the cost for the polygon mirror and motor increases.
Under such circumstances, the scanning efficiency of the polygon mirror is increased by illuminating the polygon mirror with a beam greater than the main scanning width of the polygon mirror (an overfield optical system), for example, as disclosed in Japanese Patent Application Laid-Open No. 6-143677. The scanning efficiency of the polygon can be increased by this method, but there arises a problem that spot sizes are nonuniform depending upon image heights. An effective method for relaxing this nonuniformity is to make the beam incident to the polygon mirror from in a plane made by the rotation axis of the polygon and the optical axis of the fxcex8 lens. This can minimize the nonuniformity of the spot, as compared with that in the case of incidence from other positions, and the spot becomes symmetric with respect to the image heights.
However, if the width of the incident beam is made greater than the width of the reflective facets of the polygon, there will arise a problem that reflected light from an adjacent facet appears as flare at the image plane to deteriorate the image.
An object of the present invention is, therefore, to maintain sufficient rays in the portion of the polygon mirror associated with the formation of image and efficiently intercept the reflected light from the surface not associated with the image.
In the present invention for accomplishing the above object, in order to efficiently intercept the flare, a distal end of a shield member is located within a predetermined range determined by a diameter xcfx86 of an inscribed circle to the polygon and a focal length f of the fxcex8 lens or the like.
An optical scanning apparatus according to an aspect of the invention is an optical scanning apparatus comprising a scanning mirror having a plurality of reflective facets for deflectively reflecting a first beam emitted from a laser light source and an fxcex8 lens for focusing a second beam deflectively reflected by a reflective facet of said scanning mirror, in a spot shape on an image plane,
wherein a width in a main scanning direction of the first beam incident to said scanning mirror is wider than a width of the reflective facets of said scanning mirror in the main scanning direction,
said optical scanning apparatus including a shield member for intercepting a third beam reflected by a reflective facet adjacent to the reflective facet deflectively reflecting the second beam.
In the optical scanning apparatus according to another aspect of the invention, the first beam travels through said fxcex8 lens to be incident to said scanning mirror and the first beam incident to the scanning mirror is present within a plane made by a rotation axis of the scanning mirror and the optical axis of the fxcex8 lens.
In the optical scanning apparatus according to another aspect of the invention, where the origin is set at the center of the rotation axis of said scanning mirror, an x-coordinate axis is taken along the optical axis of the first beam of incident rays, the positive direction of the x-coordinate axis is taken along a direction in which the second beam and third beam of reflected rays from the scanning mirror travel, and a y-coordinate axis along the main scanning direction, a distal end of said shield member is located in the range defined by the following equation between the reflective facet of the scanning mirror and a ray output surface of the fxcex8 lens:
xcex1x+xcex2xe2x89xa6|y|xe2x89xa6xcex1xe2x80x2x+xcex2xe2x80x2xe2x80x83xe2x80x83[Eq. 1]
xcex1=tan 2xcex80
xcex2=(sec 2xcex80)xc3x97(a cos xcex80xe2x88x922xcfx86 sin xcex80)/2
xcex1xe2x80x2=tan 2xcex81
xcex2xe2x80x2=(xe2x88x92sec 2xcex81)xc3x97(a cos xcex81+2xcfx86 sin xcex81)/2
xcex80=y0/2f
xcex81=2xcfx80/nxe2x88x92xcex80
a=xcfx86 tan(xcfx80/n),
where n is the number of facets of a polygon being the scanning mirror, f a focal length of the fxcex8 lens, y0 a maximum image height in the main scanning direction, and xcfx86 a diameter of an inscribed circle to the polygon.
In the optical scanning apparatus according to another aspect of the invention, said shield member is means for positioning said fxcex8 lens.
In the optical scanning apparatus according to another aspect of the invention, said shield member is a portion of a lens surface in a noneffective image portion of said fxcex8 lens, said portion being treated by a light-intercepting treatment.
In the optical scanning apparatus according to another aspect of the invention, said shield member is formed so as to be integral with an optical box.
In the optical scanning apparatus according to another aspect of the invention, where the origin is set at the center of the rotation axis of said scanning mirror, an x-coordinate axis is taken along the optical axis of the first beam of incident rays, the positive direction of the x-coordinate axis along a direction in which the second beam and third beam of reflected rays from the scanning mirror travel, and a y-coordinate axis along the main scanning direction, a distal end of said shield member is located in the range defined by the following equation between the fxcex8 lens and said image plane:
xcex1x+xcex2xe2x89xa6|y|xe2x89xa6xcex1xe2x80x2x+xcex2xe2x80x2xe2x80x83xe2x80x83[Eq. 2]
xcex1=2(2fxcex80 cos 2xcex80+xcfx86 sin xcex80xe2x88x92x1 sin 2xcex80)/(cos xcex80+2f cos 2xcex80)
xcex2=2fxcex80xe2x88x92[2(f+x1)(2fxcex80 cos 2xcex80+xcfx86 sin xcex80xe2x88x92x1 sin 2xcex80)/(cos xcex80+2f cos 2xcex80)]
xcex1xe2x80x2=2(2fxcex81 cos 2xcex81+xcfx86 sin xcex81xe2x88x92x1 sin 2xcex80)/(xe2x88x92cos xcex81+2f cos 2xcex81)
xcex2xe2x80x2=2fxcex81+[2(f+x1)(2fxcex81 cos 2xcex81+xcfx86 sin xcex81xe2x88x92x1 sin 2xcex81)/(cos xcex81xe2x88x922f cos 2xcex81)]
xcex80=y0/2f
xcex81=2xcfx80/nxe2x88x92xcex80
a=xcfx86 tan(xcfx80/n),
where n is the number of facets of a polygon being the scanning mirror, f a focal length of the fxcex8 lens, y0 a maximum image height in the main scanning direction, xcfx86 a diameter of an inscribed circle to the polygon, and x1 a position of a rear principal plane of the fxcex8 lens.
An image-forming apparatus according to a further aspect of the invention is an image-forming apparatus comprising the optical scanning apparatus as set forth, a photosensitive body placed on a surface to be scanned, a developing unit for developing an electrostatic latent image formed on said photosensitive body with the beam under scanning by said optical scanning apparatus, into a toner image, a transfer unit for transferring said developed toner image onto a transfer medium, and a fixing unit for fixing the transferred toner image on the transfer medium.
Another image-forming apparatus according to a further aspect of the invention is an image-forming apparatus comprising the optical scanning apparatus as set forth, and a printer controller for converting code data supplied from an external device, into an image signal and supplying the image signal to said optical scanning apparatus.